Electrophotographic (EP) laser printing employs a toner containing pigment components and thermoplastic components for transferring a latent image formed on selected areas of the surface of an insulating, photoconducting material to an image receiver, such as plain paper, coated paper, transparent substrate (electrically conducting or insulative), or an intermediate transfer medium.
There is a demand in the laser printer industry for multi-colored images. The image quality can be enhanced by a large number of approaches, including the technique which utilizes small particle developer including dry toner having an average particle size less than 5 .mu.m; see, e.g., U.S. Pat. Nos. 4,927,727; 4,968,578; 5,037,718; and 5,284,731. However, it has also been known that the electrophotographic dry toner having particle size less than 1 .mu.m is very hard to prepare due to increased specific area, and consequently, liquid toner has become one of the solutions for practical preparation of sub-micrometer xerographic developer.
Liquid toners comprise pigment components and thermoplastic components dispersed in a liquid carrier medium, usually special hydrocarbon liquids. With liquid toners, it has been discovered that the basic printing color (yellow, magenta, cyan, and black) may be applied sequentially to a photoconductor surface, and from there to a sheet of paper or intermediate transfer medium to produce a multi-colored image.
Recently, there has been an increased demand of environmental safety. The industrial response to this requirement has been the investigation of safer solvents for organic coatings. However, in the field of the photoconductor technology, the use of non-chlorinated solvents requires overcoming some challenges in the formulation of the photoconductors, because in the many photoconductor products comprising organic coatings, the best performance is easily achieved with chlorinated solvents, including the stable dispersion of organic pigments and dyes, the uniformity of the coating due to the best compatibility between the photoconductor elements, and the optimum solubility of the binder when the coating solution is made of chlorinated solvents. Thus, there is a need to combine the appropriate photoconductor elements in a non-chlorinated solvent-coating formulation so that the basic performance of the photoconductor can be achieved.
Thus, binders which exhibit satisfactory dispersion performance of the meta-stable pigment crystal forms are not always available when the non-chlorinated solvents are used.